Silicates having a framework structure containing a substituted heteroatom are expected to have characteristics, different from those of ordinary aluminosilicate zeolites, and their usage for catalytic reaction has been investigating.
As specific examples of such ferrosilicates, a ferrosilicate having platinum supported thereon has been proposed as a catalyst for isomerization of xylene in Japanese Patent 3269828; an ferrosilicate has been proposed as a catalyst for selective methylation of naphthalene compounds in Japanese Translation of PCT International Application 2004-524142 (U.S. Pat. No. 7,022,637), and a method for producing polyalkylene glycol using a ferrosilicate as a catalyst for ring-opening polymerization of a cyclic ether in Japanese patent 3477799 (U.S. Pat. No. 5,648,558).
Technology relating to reduction of nitrogen oxides using an ferrosilicate also has been investigated. Such investigations include, for example, a catalyst for purifying a nitrogen oxides-containing exhaust gas which comprises ZSM-5 type ferrosilicate having dispersed therein a co-precipitated double oxide of copper and gallium in Japanese Published Patent Application H5-305240 (U.S. Pat. No. 5,256,614); a process for reducing nitrogen oxides comprising contacting an alkali metal-exchanged ZSM-5 type ferrosilicate with a nitrogen oxides-containing exhaust gas in the presence of a hydrocarbon or an oxygen-containing compound under an excessive oxygen-containing atmosphere (Japanese Patent 2691643); a method for removing nitrogen oxides wherein exhaust gases containing nitrogen oxides, oxygen gas and, as occasion demands, sulfur dioxide gas are catalytically reacted with an ferrosilicate catalyst and a hydrocarbon reducing agent (Japanese Published Patent Application H5-154349); and, an exhaust gas purifying catalyst for reducing mainly nitrogen oxides which comprises a ferrosilicate having supported thereon at least one metal selected from platinum, palladium, rhodium and cobalt (Japanese Patent 2605956).
The ferrosilicates described in Japanese Published Patent Application H5-154349 and Japanese Patent 2605956 are synthesized using a tetrapropylammonium salt and therefore regarded as having a ZSM-5 structure as the framework structure for ferrosilicate.
With regard to catalysts for reducing nitrous oxide, proposals have also been made, which include, for example, a process for producing a catalyst comprising a β-type ferrosilicate having copper or cobalt supported thereon, which is used for direct decomposition of nitrous oxide (US Patent Application Publication 2006-0088469); a method of directly decomposing nitrous oxide using a β-type ferrosilicate; and a method of non-selectively catalytically reducing nitrous oxide using carbon monoxide as a reducing agent (Journal of Catalysis, 232 (2005) 318-334).
On the other hand, catalysts for reducing nitrogen oxides in a exhaust gas, more specifically, nitrogen oxides in excessive oxygen-containing exhaust gases such as lean-burn exhaust gas or diesel engine exhaust gas, a process for selective catalytic reduction (often referred to as “SCR”) of nitrogen oxides using ammonia is well-known (Japanese Published Patent Application H2-293021, U.S. Pat. No. 4,961,917).
However, a satisfactory ferrosilicate catalyst is not known till now, which exhibits high activity for reduction of nitrogen oxides (NOx) at a low temperature and high hydrothermal stability in a process for reducing nitrogen oxides using ammonia as a reducing agent.